I had this really great article about iridescence in feathers, written in layman's terms, but alas, I can't locate it. I hope this slight brief helps explain it some, while I keep looking for that darn article.
Pigments - Bird feather colours are produced by a variety of pigments and structural adaptations of the feathers.
* Melanins are the most common and produce black, gray, and brown. They are synthesized through oxidation of the amino acid tyrosine. Melanin granules produce colour in direct proportion to their presence in the feather; the more melanin, the darker the colour. Melanins can be found in all types of feathers, especially the major flight feathers. The pigment eumelanin produces the black, gray, and dark brown while phaeomelanin produces light brown, brick red, and dull yellow or tan.
* Carotenoids produce intense reds and yellows. They are derived exclusively from the birds diet, mainly the yellow carotene pigmets in grains, seeds, and other vegetable matter. Carotenoids are rarely seen in flight feathers but mainly in back and breast plummage and only in the contour and simiplume feather types. The pigment lutein (a xanthophyll), zeaxanthin, and beta carotene form the bright yellows. Astaxanthin, rhodoxanthin, and canthaxanthin form the bright reds.
* Porphyrins produce a range of red, brown, green, and the brown pigment of many owls. These feather pigments are related to hemoglobin and other bile pigments formed by the breakdown of hemoglobin by the liver. The most common porphyrins produce brown pigments but can also produce the bright reds and greens seen in turacos and a few other species. The pigment turacoverdin produces green while turacin (uroporphyrin) produces red, and coproporphyrin III produces brown and reddish-brown.
* Blue and green birds usually have no blue or green pigment in their plummage but rather are able to create complex patterns of reflection and refraction in the cell walls at the surface of the barbs and barbules of each feather. For example, the blue jay (Cyanocitta cristata) is not really blue but reflects only the blue light wavelengths.
* Iridescent colours result from an interaction of the microstructure of the feather and melanin granules imbedded in the barbules of each feather. In some species, iridescence results from the many laminations of keratin layers, with each reflecting different wavelengths of light. Hummingbirds have an elaborate layering of reflective melanin granules, and colours are determined by the angle of these layers relative to the viewers eye.
* Structural colours result from the modification or separation of the components of white light by the structure of the feather. In white feathers, the whole feather structure simply reflects back the whole colour spectrum. Blue is usually structural and rarely results from pigmentation. Green is usually structural but can sometimes be from a combination of yellow carotenoids and black melanins. Combinations of structural and pigment colours are common, particularly in yellow-green, green, and blue-green feathers. Iridescence is caused by the complex layering of cell walls or melanin granules in the barbules of feathers. These colours selectively absorb or reflect varying wavelengths of light and exact colours will depend on the viewers eyesight. Iridescence is primarily structural although melanin granules are almost always abundant in iridescent feathers.